I would suspect a nutrient problem -- it could just be low nitrogen. But there's also veinal chlorosis which usually indicates a magnesium deficiency or something like that. Have you fertilized the plants?

The only time I fertilize my plants is in the beginning of the growing season with fish emulsion, just to give them some boost. Other than that, I add a lot of compost, and all the plants grow fine, except I end up with this eggplant issue.

If nitrogen or magnesium is the problem, what organic fertilize would you recommend at this point?

I would mix a tablespoon or two of epsom salts in a gallon of water and water the plants with it.